The present application discloses thermochromic windows containing a film with multiple layers of alternating refractive index.
Variable transmission windows are of interest for minimizing heat load in buildings when there is direct sunlight and for maximizing daylighting throughout the day even with widely varying sunlight conditions. There is a desire for variable transmission windows to tint with a neutral color. This may require more than one chromogenic layer to achieve the required spectral coverage. Multiple layers are often needed with sunlight responsive thermochromic layers for windows and the use of multiple thermochromic layers to achieve neutral appearance of transmitted light is described in detail in U.S. Pat. No. 7,525,717, the contents of which are hereby incorporated by reference. Whether to achieve neutral coloration or if multiple layers are desired for other reasons, the thermochromic layers used in windows generally must be separated from each other in order to keep the different materials in each layer from diffusing into the other layers. If the materials intermix they could change the performance and possibly the durability of each layer. The separator layer is generally a glass or plastic layer that is impermeable to the materials in the thermochromic layers.
Preferred plastic separator layers for use in thermochromic windows are made of polyolefins like various types of polyethylene and polypropylene, cellulosics like cellulose acetate butyrate and cellulose triacetate, polyester terephthalates like polyethylene terephthalate, acrylics or polycarbonates. Plastic separator layers are typically in the form of films or sheets that are 25 microns to 500 microns thick. Particularly useful glass separators are made of soda lime or borosilicate glass with a thickness of 50 to 6000 microns.
Particularly useful thermochromic materials for use in the thermochromic layers of the present application are termed ligand exchange thermochromic, LETC, materials. LETC materials have thermochromic activity which results in a reversible change in absorbance of electromagnetic radiation as the temperature of the system is reversibly changed. That the change is reversible means that the absorbance increase as the temperature increases is the same as the absorbance decrease as the temperature decreases for a cycle of temperature increase and decrease over a given temperature range. LETC materials of interest for use in practical thermochromic systems, layers and devices are stable on repeated temperature cycling for some useful number of cycles. Particularly useful LETC materials and systems have a net increase in their ability to absorb visible and/or NIR light energy as the temperature of the system increases and a net decrease in their ability to absorb visible and/or NIR light energy as the temperature of the system decreases for temperatures within the active range of the system. The active temperature range of the system is determined by the thermodynamic properties of the LETC reactions. For many particularly useful applications, like sunlight responsive thermochromic windows, the active temperature range includes −20° C. to 100° C.
It is preferred that the electromagnetic radiation, for which absorbance changes occur, be in the ultraviolet or especially the visible and/or NIR portions of the electromagnetic spectrum. The change in light absorption on heating of the LETC systems generally results in a change from one color to another color and/or a darkening of the color of the system. If the increase in light absorption is predominantly in the NIR, the LETC system may still be very useful even though little or no visual color change occurs. However, for most preferred applications of the LETC systems or layers utilized herein, there is a net increase in the ability of the system to absorb incident visible sunlight power (or energy over time), as the temperature of the system increases from T1 to T2 and an equal net decrease in the ability of the system to absorb incident visible sunlight power (or energy over time), as the temperature of the system decreases from T2 to T1. In most cases, this means that the LETC systems become darker in color as the temperature of the system increases and lighter in color as the temperature of the system decreases. In general, there is no change in the amount of light scattered or reflected by the LETC system itself.
U.S. Pat. Nos. 7,525,717; 7,538,931; 7,542,196 and 8,018,639, which describe LETC materials, systems, layers, devices and windows are hereby incorporated into this disclosure by reference. LETC systems comprise one or more than one transition metal ion, such as Fe(II), Co(II), Ni(II) or Cu(II) ions, which experiences thermally induced changes in the nature of the complexation or coordination around the transition metal ion(s) and thereby the system changes its ability to absorb electromagnetic radiation as the temperature changes. As explained in U.S. Pat. Nos. 7,525,717; 7,538,931; 7,542,196 and 8,018,639, LETC systems employ so-called high epsilon ligands, (HεL's), and low epsilon ligands, (LεL's). Upon increasing the temperature of the thermochromic system, layer or device, one or more of the LεL's will be displaced by one or more HεL's to give a complex that absorbs more visible and/or NIR radiation. Examples of LεL's are diols, triols and certain hydroxy containing polymers like poly(vinyl butyral). Examples of HεL's are chloride, bromide, iodide and molecules that coordinate to transition metal ions through one or more than one nitrogen, oxygen, phosphorus or sulfur atom in the HεL molecule.
For the use of LETC systems in applications like variable transmission, energy saving windows, especially Sunlight Responsive Thermochromic, SRT™, windows, there is a demand for certain colors. While fixed tint windows which are gray, green, blue and bronze are in widespread use, the most desirable color, (or lack thereof), for variable tint windows is gray. This is especially true when the window is or is able to become heavily tinted as the view through a heavily tinted gray window maintains the same color rendition for objects viewed through the window as is maintained with a lightly tinted or nearly colorless window. Also it is highly desirable for the daylighting that comes in through the window to be color neutral so that people and objects illuminated by the incoming light have an acceptable appearance.
The systems disclosed here-in are particularly useful when used in combination with the window structures or constructions disclosed in U.S. Pat. Nos. 6,084,702; 6,446,402 and 7,817,328. The contents of these patents are hereby incorporated into this disclosure by reference.